The future very high temperature reactors (VHTR) are nuclear systems that shall operate at a maximum temperature of about 950 8C. Primary circuit materials thus require good creep and corrosion resistance on very long time. Use of high-strength alloys with protective coatings could significantly improve the service life of high temperature reactor components. However, coating systems are mainly designed for shorter term purposes, often under extremely aggressive atmospheres, that cannot be extrapolated to the VHTR environment. We present our first investigations on the environmental resistance of Alloy 800H coated with two different protective systems under VHTR representative conditions: NiAl(Pt)/EBPVD ZrO 2 (Y) and NiCrAl(Y)/CVD ZrO 2 (Y). Isothermal exposures were carried out up to 1000 h at 950 8C in impure helium. This specific atmosphere was shown to induce formation of a surface oxide scale together with carburisation of the bare Alloy 800H. After high temperature exposure to impure helium, the microstructure of the coated specimens has changed due to both thermal ageing and corrosion. Performances of the two coating systems are compared regarding the VHTR application.

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